Chapter 27: The Plant Body

27.1 Plant Structure and Growth: An Overview• Photosynthetic shoot system extending upward into the air and a 

nonphotosynthetic root system extending down into the soil

Growth in Plants Determinate growth common in animals Indeterminate growth in plants

Plant grows throughout life Meristems give rise to plant body Why would this be adaptive? Plasticity of growth gives some flexibility since plants cannot move

around Plants grow by: Increase in number of cells Increase in size of cells

Shoot Systems: Functions Stems, leaves, buds, flowers Highly adaptive for photosynthesis & positions flowers for pollination Vegetative (nonreproductive) shoot

Stem with attached leaves and buds Bud gives rise to extension of shoot or new, branching shoot

Reproductive shoot Produces flowers which later develop fruits containing seeds

Root Systems: Functions Usually grows below ground Anchors plant and supports upright parts Absorbs water and dissolved minerals from soil Stores carbohydrates

27.1a Cells of All Plant Tissues Share Some General Features• Organs: body structures that contain two or more types of tissues and have a 

definite form and functiono Includes leaves, stems, roots

• Tissue is a group of cells and intercellular substances that function together in one or more specialized tasks

• Primary cell walls surround the plasma membrane and cell contents(cytoplasm and organelles)

o Made of cellulose• Plasmodesmata is a cytoplasmic connection between adjacent cells that allows 

solutes such as amino acids and sugars to move from one cell to the next

o Space between primary walls and adjacent cells is filled with polysaccharide layer called middle lamella

• As a plant grows, different types of cells deposit additional cellulose and other materials inside primary wall, forming secondary cell wall

o Contain lignin: makes wallso strong and impermeable to water

• the ability of almost any cell to give rise to all other parts of a plant is totipotencyo allows plants to heal woundso one mean of asexual reproduction

27.1b Shoot and Root Systems Perform Different but Integrated Functions• a stem with its attached leaves and buds is a vegetative shoot (nonreproduuctive)• a reproductive shoot produces flowers, which later develop fruits containing seeds

27.1c Meristems Produce New Tissues throughout a Plant’s Life• determinate growth: growing to a certain size and then growth stops• indeterminate growth: growing throughout lives• meristems produce new tissues more or less continuously while the plant is alive

27.1d Meristems Are Responsible for Growth in Both Height and Girth• all plants have apical meristems: clusters of self­perpetuating tissue at the tips of 

their buds, stems, and roots• tissues that develop from apical meristems are called primary tissues, and they 

make up the primary plant body• growth of the primary plant body is called primary growth• secondary growth originates at cylinders of tissue called lateral meristems and 

increases the diameter of older roots and stems• tissues that develop from lateral meristems are secondary tissues

Primary Growth Apical meristems at root and shoot tips

Self-perpetuating clusters of cells Increases height of shoot, length of roots

Secondary Growth (in some species) Lateral meristems at root and shoot tips

Self-perpetuating cylinder of tissue Increases diameter of stems and roots

Primary Growth and Stems Primary growth produces primary plant body

Derivatives of the Apical Meristem: Three Primary Meristems Protoderm

Produces stem’s epidermis Procambium

Produces primary xylem and phloem Ground meristem

Produces ground tissue

Modifed Stems

Spatial & temporal variation in temperature• Ocean environments are less variable than those on land

Primary Growth and Leaves Leaf Primordia gives rise to leaves

27.1e Monocots and Eudicots Are The Two General Structural Forms of Flowering Plants• eudicots: trees, shrubs, non woody plants• monocots: grasses, lilies, cattails, corn, rice• monocot seeds have one cotyledon, eudicot has two

o cotyledon: leaves produced by the embryo

27.1f Flowering Plants Can Be Grouped According to Type of Growth and Lifespan• annuals are herbaceous plants in which the life cycle is completed in one growing 

season with minimal or no secondary growtho animals typically only have apical meristems

• biennials such as carrots complete their life cycle in two growing seasons and limited secondary growth occurs in some species

• perennials, vegetative growth and reproduction continue year after year

27.2 The Three Plant Tissue Systems• simple tissues have only one type of cell• complex tissues are organized with arrays of two or more types of cells• ground tissue system (makes up most of the plant body) functions in metabolism 

(including photosynthesis), storage, and support• vascular tissue system consists of xylem and phloem, which transport water and 

nutrients throughout the planto cylinders of vascular tissue are embedded in ground tissue

• dermal tissue system is a shin like protective covering for the plant body

Vascular Plant Body

3 Tissue Systems: Ground, Vascular & Dermal Organ/Tissue system: Body structure that contains two (or more!) types of tissues

and have a definite form and function Tissue: Group of one (or more!) types of cells and intercellular substances that

function together in one (or more!) specialized tasks

Primary Shoot System Consists of main stem, leaves, and buds

Plus any attached flowers and fruits Stems are adapted to provide

Mechanical support House vascular tissues Food and water storage Buds and meristems for growth

Leaves carry out photosynthesis and gas exchange

Lignin Some plant cells have “lignified” secondary cell wall

Cellulose fibres anchored with lignin: stronger and more rigid Creates waterproof barrier (hydrophobic) Resistant to decay and attack by microbes

Plant Tissue System: Form & Function 3 Tissue Systems: Ground, Vascular & Dermal Ground tissues are all structurally simple but exhibit important differences Vascular tissues are specialized for conducting fluids The dermal tissue system protects plant surfaces

Ground, Vascular, and Dermal Tissues

27.2a Ground Tissues Are All Structurally Simple but Exhibit Important Differences• 3 types of tissues: parenchyma, collenchyma, sclerenchyma

• parenchyma: soft primary tissueso makes up bulk of primary growth of roots, stems, leaves, flowers, fruitso thin primary wallo permeable to watero large air spaceso storage, secretion, photosynthesiso alive and metabolically active when matureo Thin primary cell walls, pliable and permeable to water, often roundo Air spaces (gas exchange, buoyancy)o Specialized for storage, secretion, photosynthesis

They are found in the cortex and pith of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds.

o Metabolically active when matureo Capable of additional cellular division (meristematic) if stimulated

• collenchyma: flexible supporto strings in celeryo strengthen plant partso cellulose and pectino walls stretch as the cell enlarges, growing organso alive and metabolically active, continue to synthesize primary wall layers 

as the plant growso Thicker (uneven) primary cell walls (pectin) – especially at the cornerso Elongated cells in strands or sheath like cylindero Not uniformly thick wallso Strengthen plant parts still elongating

Found adjacent to outer growing tissues and the vascular cambiumo Metabolically active & meristematic

Additional growth stimulated by mechanical stress• sclerenchyma: rigid support and protection

o Thick, uniform, secondary cell walls (hemicelluslose & lignin)o No air spaces between them, large amount of strengtho Adding lignin chokes off plants, and thus these cells are terminalo Dead at maturity as cut off from rest of organismo Found in stems, leaf veins and make up the hard outer covering of seeds &

nuts.o Two major types

Sclereids (protective casings) • Cells are irregular in shape. Commonly found in fruit and

seeds. Fibres (support)

• Cells are often needle-shaped with pointed tips – some elasticity

Ground Tissues

27.2b Vascular Tissues Are Specialized for Conducting Fluids• xylem: transporting minerals and water

o conducts water and dissolved minerals absorbed from the soil upward from a plants roots to the shoot

o types of conducting cells: tracheid’s

• elongated with tapered, overlapping ends• strong secondary walls to keep plants from collapsing when 

water becomes scarce• water can move between through pits

o seeps laterally between cells vessel members

• shorter, wider cells joined end to end in tubelike columns called vessels

• have pits• as they mature, enzymes break down portions of their end 

walls, producing perforations• moves water more efficiently

• phloem: transporting sugars and other solutes

o transports solutes and sugars made in photosynthesis through the plant body

o main conducting cells are sieve tube members connected end to end end walls called sieve plates, are studded with pores immature sieve tube members contain the usual plant organelles, 

but the cell nucleus and internal membranes in plastids break down, mitochondria shrink, and cytoplasm is reduced to a think layer lining the interior surface of the cell wall

• specialized parenchyma cells known as companion cells are connected to mature sieve tube members by Plasmodesmata

Vascular Tissue: Xylem• Conducts water and dissolved minerals• Thick, lignified secondary walls• Dead when functional

Types of Xylems: Tracheids and Vessel MembersTracheids

• Elongated, tapered, overlapping ends• Lateral connections through pits

Vessel members• Shorter, tubelike columns• Lateral connections through pits and perforations• Vessel members are better at quickly moving water, more easily blocked by air

bubbles in water

Vascular Tissue: Phloem• Conduct sugars and other solutes • Living* when functional

Sieve Tube Members Joined end to end in sieve tubes Sieve tube cells assisted by “companion cells”

Parenchyma cells that load and unload organic compounds into sieve tube End walls (sieve plates) studded with pores

27.2c The Dermal Tissue System Protects Plant Surfaces

• epidermis covers primary plant body in single continuous layer or multiple layers of packed cells

• external surface of epidermal cell walls is coated with waxes that are embedded in cutin, a network of chemically linked fats

• epidermal cells secrete cuticle which resists water loss and helps protect against attacks by microbes

• guard cells contain chloroplasts and so can carry out photosynthesiso pore between guards is stomata (water vapor, co2, and oxygen cross this) 

(guard cells regulate open and closing of these)• trichomes give stems or leaves of some plants a hairy appearance

o exude sugars to attract pollinators• root hairs are extensions of outer wall of root epidermal cells (also trichomes)

o absorb much of plants water and minerals from the soil Epidermis covers primary plant body

Cuticle layer restricts water loss Pairs of guard cells in leaf epidermis create stomata for gas exchange Epidermal specializations Trichomes (hairs) Absorbent root hairs

Periderm Cork, cork cambium, and secondary cortex

27.3a Stems Are Adapted to Provide Support, Roots for Vascular Tissues, Storage, and New Growth

• 4 main functions:

1. stems provide mechanical support, generally along a vertical axis for body parts involved in growth, photosynthesis, reproduction

2. stems house the vascular tissues which transport products of photosynthesis, water, and dissolved minerals, hormones  and other substances throughout the plant

3. stems are often modified to store water and food4. buds and specific stem regions contain meristematic tissue that gives rise 

to new cells of the shoot• modular organization of a stem

o plant stem develops in a pattern that divides the stem into modules, each consisting of a node and an internode

o node is a place on the stem where one or more leaves are attached; region between is an internode

o new primary growth occurs in buds – terminal bud at the apex of the main shoot, and axillary buds – produce branches at the point where leaves meet the stem

• in eudicots, most growth in a stem’s length occurs directly below the apical meristem as internode cells divide and elongate

o internode cells nearest the apex are most active, so most visible growth occurs at ends of stems

• in monocots, the upper cells of an internode stop dividing as the internode elongates ad cell divisions are limited to a meristematic region at the base of the internode

• terminal buds release a hormone (auxin) that inhibits the growth of nearby lateral buds, a phenomenon called apical dominance

o flow of hormone signals then dwindles to a level low enough that lateral buds begin to grow

• in nature, apical dominance is an adaptation that directs the plant’s resource into growing up toward the light

• shoot apical meristem is a dome­shaped mass of cells at the tip of shoots, surrounded by developing leaves

• differentiating cells give rise to 3 primary meristems: protodern, procambium, ground meristem

o primary meristems are also responsible for elongation of the plant bodyo protodern, a meristem that gives rise to the stems epidermis is the 

outermost layer of the shoot tipo inward from the protoderm is the ground meristem which will give rise to 

ground tissueo procambium produces vascular tissues, is sandwiched between ground 

meristem layers long and thin, spatial orientation foreshadows the function of the 

tissues they produce inner procambium cells give rise to xylem and outer procambial 

cells give rise to phloem

in plants with secondary growth, a thin region of procambium between the primary xylem and phloem remains undifferentiated. Later it will give rise to lateral meristem

vascular bundles are cylinders of primary xylem and phloem that are sometimes wrapped in sclerenchyma

• eudicot stems have vascular bundles arranged in a circle that separates ground tissues from stems

• monocot stems also have vascular bundles, but these are scattered throughout the ground tissue, so distinct pith and cortical regions do not form

• arrangement of vascular bundles in a plant ultimately depends on the number of branch points to leaves and buds and on the number and distribution of leaves

o stem modifications bulb is a modified shoot that consists of a bud with fleshy leaves tubers are stem regions enlarged by the presence of starch­storing 

parenchyma cells rhizomes are long underground stems that produce new shoots 

when existing ones are pulled out stolon’s are slender stems that grow along soil surface

Vascular Tissues Organized into vascular bundles

Primary phloem and xylem in each bundle Wrapped in sclerenchyma for support

Lengthwise through parenchyma

StemsOrganized into modular segments

Nodes: Where leaves are attached Internodes: Between nodes Terminal bud: At apex of main shoot Lateral buds: In leaf axils, produce branches

27.3b Leaves Carry Out Photosynthesis and Gas Exchange• leaf morphology and anatomy

o leaf blade provides large surface area for absorbing sunlight and co2• petiole hold leaves away from the stem and prevent individual leaves from 

shading one another (longer and narrower in monocots)• leaf primary growth and internal structure

o leaf primordial gives rise to leaves happens when shoot apical meristem divides and has bumps on 

sideso below epidermis of leaf is mesophyll: ground tissue composed of loosely 

packed parenchyma cells that contain chloroplasts palisade

• more chloroplasts• compact columns• smaller air spaces between them

spongy• underside of the leaf• large air spaces• enhances uptake of co2 and release of o2 in photosynthesis• contains collenchyma and sclerenchyma

o below mesophyll is another cuticle covered epidermal layero vascular bundles form a lacy network of veins throughout the leafo eudicot leaves typically have a branching vein pattern, monocot has 

parallel veins

Two Major Classes of Flowering Plants Monocots

One cotyledon (seed leaves in plant embryos) Grasses, lilies, cattails, corn, rice

Eudicots Two cotyledons Poplars, willows, oaks, cacti, roses, poppies, sunflowers, garden beans,

peas

Eudicot Vascular Bundles Form a circle

Outer cortex, inner pith

Monocot Vascular Bundles Scattered throughout ground tissue of stem

Monocots v Eudicots

 

27.4a Taproot and Fibrous Root Systems• taproot: single main root adapted for storage, with branching roots called lateral 

roots• fibrous root system: several main branches form dense mass of smaller roots

o adapted to gather water and nutrients from upper layers of the soilo prevent erosion

• adventitious roots refers to any structure arising at an unusual location

27.4b Root Structure is specialized for Underground Growth• root cap is produced by meristem, and surrounds and protects meristem as root 

elongates through soil• zones of primary growth in roots

o root apical meristem and actively dividing cells behind it form the zone of cell division

o root capo quiescent centero merges into zone of elongation

cells become longer as their vacuoles swell hydraulic elongation

o zone of maturation• tissues of the root system

o ground meristem gives rise to roots cortex

often contains air spaces that allow oxygen to reach all of the living root cells

o in flowering plants, the outer root cortex gives ride to exodermis, a thin band of cells that may limit water losses from roots and help regulate the absorption of ions

o innermost layer of root cortex is the endodermis, a thin selectively permeable barrier that helps control the movement of water and dissolved minerals into the stele

o the outermost layer of stele, between the endodermis and the phloem, is the pericycle, consisting of one or more layers of parenchyma cells that have retained the ability to function as meristem

pericycle initiates the formation of lateral roots in response to chemical growth regulators

lateral roots grow out through the cortex and epidermis, providing enzymes that help break down the intervening cells

distribution frequency of lateral root formation partly control the overall shape of the root system and the tent of the soil area it can penetrate

Lateral Root Formation Root primordia in pericycle form lateral roots

Secondary Growth: An Adaptive ResponsePlants with taller stems or wider canopies can intercept more light energy from sun

With great energy supply for photosynthesis, have metabolic means to increase root and shoot systems

Therefore: Better able to acquire resources Better able to ultimately reproduce successfully

Primary and Secondary Growth Seconday Growth increases girth of roots and stems Vascular cambium

Produces secondary xylem (wood) and secondary phloem Cork cambium

Produces cork, part of protective bark

Vascular Cambium Fusiform initials generate secondary xylem and phloem Ray initials produce horizontal water transport channels called xylem rays

Woody Stem Structure Heartwood, sapwood

Support of Growth Exogenous trees (all conifers, and almost all broadleaf trees), grow by the addition

of new wood outwards, immediately under the bark. As an exogenous tree grows, it creates growth rings as new wood is laid down

concentrically over the old wood Endogenous trees, mainly monocotyledons (e.g., palms and dragon trees), but also

cacti, grow by addition of new material inwards.

Root Secondary Growth

27.5 Secondary Growth• in plants that have secondary growth, older stems and roots become more massive 

and woody through two types of lateral meristemso vascular cambium

produces secondary xylem and phloemo cork cambium

produces cork secondary tissue that replaces original epidermis of the plant

27.5a Vascular Cambium Gives Rise to Secondary Growth in Stems• takes place as the cells of the vascular cambium divide• division of the vascular cambium produces secondary xylem to ward the inside of 

the stem and secondary phloem to warm the outside of the stem• mass of the secondary xylem inside the ring of vascular cambium increases, 

forming the hard tissue known as wood• outside vascular cambium, secondary phloem cells are also added each year• walls of cork cells contain lignin and thick layers of suberin, a waxy substance 

that is impermeable to water and gaseso cork cells are dead at maturity

• cork cambium and phelloderm make up the periderm• bark encompasses all the tissues outside the vascular cambium• lenticels are tissues with abundant air spaces that allow the exchange of oxygen 

and co2 between the living tissues and the outside air• heartwood is the center of its older stems and roots, and dry tissue that no longer 

transports water and solutes and is a storage depot for some defensive compounds• sapwood is secondary growth located between heartwood and the vascular 

cambium• growth rings provide information on past climates• dendroclimatologist use tree rings and other biological information to reconstruct 

past environments